This invention pertains generally to gas turbines and more particularly to high temperature gas turbines that employ a cooling medium to maintain the components within the working gas flow path within acceptable temperature limits.
In most industrial combustion turbines, ambient air is drawn into the intake of a compressor, compressed and delivered to a combustion where it is combined with a fuel, ignited and transported through a transition member to a turbine wherein the working gas is expanded to produce mechanical energy. The compressor and turbine rotor are typically coupled to a common shaft so that rotation of the turbine rotor drives the compressor. Similarly, in power plant applications, the turbine rotor is also connected to a generator rotor to drive the generator to produce electricity.
A typical combustion turbine system 10 portion of such a power plant is illustrated in FIG. 1. Generally, the combustion turbine is made up of a compressor section 12, a combustion section 34 and a turbine section 41. The compressor section 12 is made up of a plurality of stationary vanes 14 supported by the outer housing and rotating blades 16 which are mounted on a common shaft 18. Each rotating blade row followed by a stationary vane row constitutes a compressor stage. FIG. 1 shows sixteen compressor stages. The compressor shown is also equipped with an inlet guide vane (IGV) 9, and an outlet guide vane (OGV) 29. The compressor is also arranged to form compressor bleed ports 22, 24 and 26 for bleeding compressed air for cooling high temperature turbine components. Ambient air 13 is introduced through an inlet 11 and successively compressed in each compressor stage, and it flows by all the bleed ports 22, 24 and 26, and the rest of compressor stages 28, and OGV 29 after which the compressed air travels through annular diffuser 30 to compressed air plenum 32 which surrounds the combustion 34 and transition member 36. A portion of the compressed air 13 can be diverted from each of the bleed ports for cooling turbine components as discussed above. A portion of the compressed air, as shown by the arrow, reverses direction in the plenum 32 and travels between the combustion housing 38 and the combustion shell 40 where it is directed to a combustion inlet and combined with fuel introduced through the nozzle inlets 42. The combined fuel and compressed air is burned in the combustion 34 to create a working gas which is directed through the transition member 36 to an inlet 44 to the first stage of the turbine 41. The turbine section 41 is made up of a serial arrangement of stationary vanes 52 and rotating blades 54. The rotating blades are supported by a common rotor system 56 and the vanes and blades are arranged in serial stages 44, 46, 48 and 50, which form the first through fourth stages of the turbine section. The working gas exiting the transition member 36 then expands through the stages 44, 46, 48 and 50 causing rotation of the blades 54 which in turn impart mechanical, rotational power to the rotor system 56. The turbine rotor system 56 is connected to the compressor shaft 18 so that rotation of the turbine rotor system 56 drives the compressor 12. Normally, in power plant applications, the rotor system 56 is coupled to the rotor of a generator to drive the generator to create electricity. The working gas ultimately is exhausted at the exit to the turbine 58 and directed through an exhaust stack to the ambient atmosphere.
It is generally desirable to have the turbine work at the highest efficiency possible. It is also known that the higher the temperature of the working gas, the higher the efficiency of the turbine. However, the upper temperature that the working gas can practically function at is limited by the temperature characteristics of the materials that it interfaces with. In addition, the higher the temperature of the combustion process and working gas, the more pollutants such as NOx that are created. Stringent environmental restrictions require such pollutants to be kept below a minimal level. These competing interests have been addressed by leaning out the combustion mixture to reduce flame temperature while maintaining an overall higher average thermal output and cooling the various components interfacing with the working gas flow path.
A system which has been employed for cooling the various turbine components is illustrated in FIG. 1. The cooling system shown is an open loop cooling system wherein compressed air is introduced into the various components and after traversing a cooling path within a component, is exhausted into the working gas within the turbine, providing power augmentation. To assure that the working gas does not backflow through the cooling system, the pressure of the compressed air has to be greater than that of the working gas at the point at which the cooling air is introduced into the working gas flow path. In this regard, air 60 is bled from the first bleed port 22 of the compressor and introduced at the fourth stage 50 of the turbine to cool the turbine stationery components before being introduced into the working gas flow path around the rotating blades 54 at a point where the working gas is at its lowest pressure among the turbine stages. Similarly, air 62 is bled from a second bleed port 24 of the compressor 12 and introduced at the third stage 48 of the turbine 41; and air 64 is bled from the third bleed port 26 of the compressor 12 and introduced at the second stage 46 of the turbine. The compressed air exiting the compressor outlet 30 is used to cool the combustion shell and liner and the transition member before being introduced into the working gas path within those components. The air 66 exiting the compressor outlet 30 is also used to cool the first stage of the turbine and further diverted, as represented by reference character 68, to cool the internal components of the rotor and the rotating blades 54, before being introduced into the working gas flow path. In this manner, the internal components of the combustion, transition and turbine are able to accommodate higher temperatures for greater overall turbine efficiency. However, diverting compressed air from the compressor for cooling has a negative affect on the efficiency of the operation of the turbine in that there is less air available for combustion and to be introduced at the first stage of the turbine for power conversion from thermal power to mechanical power. Accordingly, it is desirable to find another means of cooling the turbine components interfacing with the working gas flow path that does not require or minimizes the diversion of air from the compressor. Accordingly, it is an object of this invention to provide a system that minimizes the use of compressed air for cooling a turbine""s internal components along the working gas flow path.
The instant invention takes advantage of a nitrogen source in a power plant to supply nitrogen in lieu of compressed air to at least a portion of the cooling circuit in a combustion turbine. In a preferred arrangement, the nitrogen is supplied from an air separation unit that separates oxygen from the nitrogen in the air for use in an integrated gasification combined cycle (IGCC) plant. Upon startup of the plant, air is initially supplied to the cooling circuit of the combustion turbine until the nitrogen becomes available. In one preferred arrangement, a cooling control system monitors the availability of nitrogen and controls valves within separate legs of the combustion turbine cooling circuit to supply the nitrogen in lieu of the compressed air sequentially, one leg at a time as the nitrogen becomes available. In a preferred scheme, the cooling leg corresponding to the hottest turbine components is supplied nitrogen first along with compressed air until sufficient nitrogen is available to replace the compressed air in that cooling leg completely. The control system next adds nitrogen to the next successive leg corresponding to the next hottest component, one leg at a time until preferably the nitrogen completely supplants the cooling air in the second, third and fourth stage of the turbine stator and the turbine rotor.
In the preferred arrangement, the cooling system controller also monitors the temperatures of the components being cooled and regulates the volume of nitrogen supplied to those components to maintain the temperature in an acceptable range.